Fundamental aspects of Pickering emulsion stabilisation

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Abstract

Much research has been carried out in recent years on Pickering emulsions, but
understanding of the underlying physics requires considerable strengthening. This
thesis seeks to address several fundamental aspects by presenting the results of
recent experimental work.
This work has focused on a model oil-in-water emulsion system stabilised by
fluorescent colloidal silica particles and using a mixture of dodecane and isopropyl
myristate as the oil phase. The phase behaviour of the particle dispersions has
been altered using sodium chloride and sodium iodide, whilst sodium hydroxide
and hydrochloric acid have been used to adjust the pH of samples. Comparisons
are also made to emulsions stabilised by commercially available fumed silica.
Conventionally, it was assumed that a weakly flocculating particle dispersion is
required in order to generate a stable Pickering emulsion. It is shown in this
work, however, that in some circumstances a weakly flocculating dispersion leads
to the least stable emulsion. It is therefore argued that a more nuanced view
of Pickering stabilisation is required, taking into account the factors affecting
whether particles will adsorb to the interface during emulsification.
Very recently it has begun to be suspected that Pickering emulsions sometimes
aggregate due to the sharing of particles between two droplets, an effect known
as bridging. In this thesis it is also shown that particle bridges can form in
Pickering emulsions at high shear, and that they can subsequently be broken by
low shear or by modifying the particle wettability. For the first time, electron
microscopy has been used to provide direct evidence of droplets sharing particles.
A simple theoretical model is developed, based on collisions between partially
coated droplets, which captures the trends observed experimentally. It is argued
that particle bridging may have been overlooked in the literature, and that the
shear history of emulsions is a crucial determinant of subsequent behaviour.
The deaggregation of bridged emulsions has been studied using a novel method
where two different colours of particles are used. By starting with two emulsions
which are bridged, each stabilised by a different colour of particle, and then using
confocal microscopy to study them as they are mixed together and deaggregate,
the processes involved in deaggregation can be elucidated. These experiments
have also shown, for the first time, the dynamic nature of particles in Pickering
emulsions; particles transfer readily between droplets when the samples are placed
on a roller bank. It is found that a period of unbridging and rebridging takes
place prior to deaggregation of the emulsions, and the timescale of deaggregation
can be tuned by varying the particle wettability.
The two-colour method has also been applied to the study of Pickering emulsions
which are repeatedly sheared. It is found that limited coalescence is not reestablished
simply by re-applying the shear rate which was used in the initial
emulsification. This behaviour is attributed to the presence of an elastic shell of
particles at the interface, which inhibits droplet breakup, and is in contrast to that
of surfactant-stabilised emulsions, where increasing the stabiliser concentration
makes droplets more liable to deform and breakup.
Finally, a short study has been carried out attempting to increase the scale of
the experiments presented in this thesis to sample volumes of approximately one
litre. This study has demonstrated the relevance of particle bridging to industrial
emulsification processes.
Overall, experiments with carefully controlled model Pickering emulsions, including
those using two colours of particles, have revealed the fundamental workings
of these arrested systems.